This invention relates to a method and apparatus for assembling semiconductor chips or "dice" into devices such as LED lamps, detectors, displays, optoisolators, optoswitches and arrays of emitters and/or detectors. The semiconductor components described are light emitting chips and light sensing detectors, but the invention is also applicable to other components such as integrated circuit chips.
Typically, light emitting diodes ("LED"s) and detectors are produced from large wafers containing thousands of individual emitter diodes or detector transistors. The wafers are mounted on a membrane and divided into dice (each die usually comprising a single LED or detector) by sawing, or scribing and breaking. The membrane is stretched to separate the dice, and an automatic die attaching machine is used to pick up the dice one at a time and place them on a leadframe, which has been pre-coated one unit at a time with conducting epoxy to make one of the required electrical connections to the die. Typical speed for such a machine is 3,000 units per hour and leadframe spacing is typically 3 per inch. The pick-up mechanism on the die attaching machine usually requires automatic pattern recognition, since after the membrane is stretched, the exact location of the dice is lost.
After the die attaching operation, the leadframe is moved to a die bonding machine and a fine wire is attached between the top metal contact on the die and the second lead on the leadframe. This operation is typically performed at a rate of 3,000 to 5,000 units per hour, limited by the large spacing between leadframes and the precise position of the bond. The leadframe is then inverted and inserted into a mold and a thermosetting epoxy is cast around the leadframe. The epoxy is cured by heating in an oven for approximately 30 minutes. After molding, the leadframes are cut apart into individual devices (e.g., lamps or detectors), tested, and binned according to brightness, color or other significant characteristics.
Displays differ in construction from the above in that many dice are placed on a single leadframe or circuit board. This is generally a slower and more costly process to perform using automatic equipment since the distance between dice is larger and positioning must be precise. The cost increases and speed decreases for automatic equipment as distance and precision of placement increases.
Two leadframes rather than one are usually used in the manufacture of optoisolators--one for the emitter and one for the detector. The detector frequently requires more than two contacts. The two leadframes are positioned in proximity to each other so that radiation from the emitter can impinge on the detector. The leadframes are usually molded twice--first with a transparent material and then with an opaque material. Optoisolator manufacturing is limited, as is display manufacturing, by the requirement for precise spacing of widely spaced dice on leadframes.